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Work done by a constant torque equation DOESN'T give me angular speed Help!

  1. Nov 19, 2012 #1
    Number15.png

    What is given:

    Torque: τ = 34.5 N * m
    Inertia: I = 0.130 kg * m2
    Angular speed: ω = ?
    Work done by a constant Torque: τΔθ = (1/2)Iωf2 [STRIKE]- (1/2)Iωi2[/STRIKE]
    Angular displacement: Δθ = 17.1 rev/s = 107.442 rad/s


    τΔθ = (1/2)Iωf2
    (34.5)(107.442) = (0.5)(0.130)ωf2
    3706.765 = 0.065ωf2
    57027.156 = ωf2
    238.8 = ω

    It's still wrong :(
     
    Last edited: Nov 19, 2012
  2. jcsd
  3. Nov 19, 2012 #2
    Δθ is 17.1 revolutions. You need to convert that to radians.
     
  4. Nov 19, 2012 #3

    Doc Al

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    Staff: Mentor

    Make sure your angular displacement is in radians, not revolutions.
     
  5. Nov 19, 2012 #4
    Change 17.1rev's to ... radians's.
     
  6. Nov 19, 2012 #5

    Doc Al

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    Staff: Mentor

    3x should be enough :biggrin:
     
  7. Nov 19, 2012 #6
    Wait what? :(
     
  8. Nov 19, 2012 #7
    I just did and it's still wrong
     
  9. Nov 19, 2012 #8

    Doc Al

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    Staff: Mentor

    Show what you did.
     
  10. Nov 19, 2012 #9
    If you show your work help may be given.
     
  11. Nov 19, 2012 #10
    B]What is given:[/B]

    Torque: τ = 34.5 N * m
    Inertia: I = 0.130 kg * m2
    Angular speed: ω = ?
    Work done by a constant Torque: τΔθ = (1/2)Iωf2 [STRIKE]- (1/2)Iωi2[/STRIKE]
    Angular displacement: Δθ = 17.1 rev/s
    Conversion (17.1 x 2 x π) rad/s = 107.442

    τΔθ = (1/2)Iωf2
    (34.5)(107.442) = (0.5)(0.130)ωf2
    3706.765 = 0.065ωf2
    57027.156 = ωf2
    238.8 = ω
     
  12. Nov 19, 2012 #11
    But I just did o_O
     
  13. Nov 19, 2012 #12

    Doc Al

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    Staff: Mentor

    Looks like they want the speed in rev/s, not rads/sec.
     
  14. Nov 19, 2012 #13
    Sooo...I don't convert angular displacement?
     
  15. Nov 19, 2012 #14

    Doc Al

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    Staff: Mentor

    Convert your answer for ω from rad/s to rev/s.
     
  16. Nov 19, 2012 #15
    τΔθ = (1/2)Iωf2
    (34.5)(107.442) = (0.5)(0.130)ωf2
    3706.765 = 0.065ωf2
    57027.156 = ωf2
    238.8 rad/s = ω

    238.8/2π rev/s = ω
    38 rev/s = ω

    Awesome :eek:
    Thanks!
     
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